1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated transmission which is connected to an engine via a fluid torque converter with a lockup clutch and which is mainly used in an automatic transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
An automatic transmission can normally be operated for selective switching among parking range, reverse range, neutral range, automatic transmission (Drive or D) range, and low-speed holding range. In the automatic transmission range, overrotation of the output side is allowed via a one-way clutch at the time of establishment of a low-speed transmission train. In the low-speed holding range, a hydraulic engaging element for engine braking which operates to lock the one-way clutch is engaged to thereby establish a low-speed transmission train in a condition in which the overrotation of the output side is not allowed, i.e., in a condition in which the engine braking can be applied or effected.
Conventionally, there is known an apparatus in which a plurality of hydraulic engaging elements provided in a transmission are controlled by a plurality of solenoid valves corresponding to these hydraulic engaging elements. In such an apparatus, since the hydraulic engaging element for engine braking is not engaged in the automatic transmission range, it has been considered advantageous to utilize the solenoid valve which controls the hydraulic engaging element for engine braking, to also control that lockup clutch of the fluid torque converter which becomes necessary in the automatic transmission range.
For example, in Japanese Published Unexamined Patent Application No. 278076/1990, an apparatus is known in which there is provided a shift valve for engine braking for switching on and off a hydraulic engaging element for engine braking, and a lockup shift valve for switching on and off a lockup clutch. In the low-speed holding range, the lockup shift valve is restrained to a switched off position (OFF position) and also the shift valve for engine braking is switched by the output pressure of a solenoid valve to thereby control the switching on and off of the hydraulic engaging element for engine braking. On the other hand, in the automatic transmission range, the shift valve for engine braking is restrained to the OFF position, and the lockup shift valve is switched by the output pressure of the same solenoid valve as above to thereby control the switching on and off of the lockup clutch.
In the above-described apparatus, when the output pressure of the solenoid valve remains a high pressure due to failure or trouble of the solenoid valve, the lockup clutch will always be switched on in the automatic transmission range. As a result, engine stalling is likely to occur at the time of starting a vehicle.
Though not provided with a one-way clutch, there is known a transmission of the following construction. Namely, there is provided a changeover valve for selectively connecting an oil passage on the output side of a solenoid valve which controls a hydraulic engaging element for a low-speed transmission train to an oil passage for controlling the hydraulic engaging element and to an oil passage for controlling the lockup clutch. The changeover valve is switched by the hydraulic oil pressure to be generated at the time of establishing a high-speed transmission train to thereby connect the oil passage for controlling the lockup clutch to the oil passage on the output side of the solenoid valve. The operation of the lockup clutch is thus controlled by the output pressure of the solenoid valve (see the specification of U.S. Pat. No. 4,936,166).
By applying this kind of art, the following control may be considered. Namely, there is provided a changeover valve between an oil passage to be connected, in the automatic transmission range, to the output side of a solenoid valve for controlling the hydraulic engaging element for engine braking and an oil passage to be communicated with a control circuit portion of a lockup clutch. The changeover valve is switched by the hydraulic oil pressure to be generated at the time of establishing a transmission train other than the lowest-speed transmission train that can be established in the automatic transmission train, i.e., the first-speed transmission train. Both the above-described oil passages are thus communicated with each other to thereby control the operation of the lockup clutch by the output pressure of the solenoid valve.
According to this arrangement, even if the output pressure of the solenoid valve may remain a high pressure, the changeover valve is in a condition of shutting off the communication between both the oil passages at the time of starting of the vehicle by establishing the first-speed transmission train. Therefore, the lockup clutch will not be switched on and consequently the engine stalling will not occur. In this arrangement, however, that hydraulic oil pressure to be operated on the changeover valve which is generated at the time of establishing a transmission train other than the first-speed transmission train varies at the time of speed changing. Therefore, there is a possibility that the changeover valve may temporarily be switched to the condition in which the communication of both the oil passages is shut off at the time of speed changing while the vehicle is running at a speed above the second-speed transmission train, resulting in an unstable control of the lockup clutch.
In view of the above-described points, the present invention has an object of providing a control apparatus for a hydraulically operated transmission in which the engine stalling at the time of starting the vehicle is prevented and in which the lockup clutch can be stably controlled.